The invention applies advantageously, but not exclusively, to cellular mobile telephones with a direct conversion architecture. To perform a direct uplink conversion in a radiofrequency transmission subsystem, two orthogonal radiofrequency signals having a frequency equal to the wanted output frequency are normally used. These two orthogonal transposition signals are multiplied by a complex baseband signal, represented by two channels I and Q (channel in phase and channel in phase quadrature). The two signals transposed to the output frequency are then summed to provide a resultant output signal.
Now, such a method presents major drawbacks. In practice, normally, the two transposition signals result from an integer frequency division of an oscillator signal produced by a phase-locked loop, and the frequency of which is equal to an integer multiple of the wanted output frequency. Now, since the frequency of the oscillator signal delivered by the phase-locked loop is equal to K times the wanted output frequency, this frequency of the oscillator signal coincides with the Kth harmonic of the output frequency. And, in the case of coupling between the signals transposed to the output frequency and the local oscillator signal of the phase-locked loop, this Kth harmonic generates a phenomenon commonly known to those skilled in the art as “pulling”, which degrades the quality of the radiofrequency signal.
Moreover, when K is equal to 2, an error in the duty cycle of the signal delivered by the phase-locked loop provokes a phase error between the two transposition signals which are no longer orthogonal. The result is then a further degradation of the quality of the radiofrequency signal after transposition. This problem can be avoided by choosing a factor K equal to 4, that is, by doubling the frequency of the phase-locked loop. However, this results in an increase in consumption.
This “pulling” problem can prove so crucial, in particular in single-chip devices, that the direct conversion is then replaced by a heterodyne conversion, resolving the problem of “pulling”, but leading to far more complex architectures.